1. Field of the Invention
The present invention relates to a plating apparatus and a plating method for carrying out plating of a surface of a plating object (substrate), such as a semiconductor wafer, and more particularly to a plating apparatus and a plating method useful for forming a plated film in fine interconnect recesses or holes or resist openings, provided in a surface of a semiconductor wafer, or for forming bumps (protruding electrodes), which are for electrical connection to, e.g., electrodes of a package, on a surface of a semiconductor wafer.
2. Description of the Related Art
It is common practice, e.g., in TAB (tape automated bonding) or flip chip to form protruding connection electrodes (bumps) of gold, copper, solder or nickel, or of multiple layers of such metals at predetermined portions (electrodes) of a surface of a semiconductor chip, having interconnects formed therein, so that the semiconductor chip can be electrically connected via the bumps to electrodes of a package or TAB electrodes. There are various methods available for the formation of bumps, such as electroplating, vapor deposition, printing and ball bumping. Of these, electroplating, which can form fine bumps and can be performed in a relatively stable manner, is most commonly used as the I/O number of a semiconductor chip increases and the electrode pitch becomes smaller.
A high-purity metal film (plated film) can be obtained with ease by electroplating. Further, electroplating can not only form a metal film at a relatively high rate, but can also control a thickness of the metal film relatively easily. In the formation of a metal film on a semiconductor wafer, the in-plane uniformity of a thickness of the metal film is strictly required in order to attain high-density packaging, high performance and high yield. When electroplating is used to form a metal film, the distribution of metal ion feed rate and the distribution of electric potential in a plating solution can be made uniform. It is therefore expected that electroplating will be capable of obtaining a metal film excellent in the in-plane thickness uniformity.
Among plating apparatuses that employ a so-called dipping method, a plating apparatus is known which comprises a plating tank for holding therein a plating solution; a substrate holder for holding a substrate (plating object) vertically with its peripheral portion watertightly sealed thereby; an anode vertically held by an anode holder and disposed so as to face the substrate in the plating tank; a regulation plate of dielectric material, having a central hole, disposed between the anode and the substrate; and a paddle disposed between the regulation plate and the substrate for stirring the plating solution (see, e.g., International Publication No. WO 2004/009879 pamphlet, patent document 1).
In operation of the plating apparatus described in the patent document 1, the anode, the substrate and the regulation plate are immersed in the plating solution in the plating tank while the anode is connected to an anode of a plating power source and the substrate is connected to a cathode of the plating power source via conducting wires, and a predetermined plating voltage is applied between the anode and the substrate, thereby depositing a metal and forming a metal film (plated film) on the surface of the substrate. During the plating, the plating solution is stirred with the paddle disposed between the regulation plate and the substrate so as to uniformly supply a sufficient amount of ions to the substrate, thereby forming a metal film having a more uniform thickness.
According to the plating apparatus of the patent document 1, the distribution of electric potential in the plating tank can be controlled by the regulation plate, having a plating solution passage in a cylindrical body, disposed between the anode and the substrate disposed opposite the anode. This enables control of a thickness distribution of a metal film formed on the surface of the substrate.
Further, a plating apparatus has been proposed which, by minimizing the distance between a regulation plate and a plating object, both immersed in a plating solution in a plating tank, can make the distribution of electric potential more uniform over an entire surface of the plating object, thereby forming a metal film having a more uniform thickness (see, e.g., Japanese Patent Laid-open Publication No. 2001-329400, patent document 2).
These days, in order to increase productivity, there is a strong demand to shorten plating time taken for forming a plated film having a given thickness to about ⅔ of the conventional plating time. In order to form a plated film having a given thickness and a given plating area in a shorter time, it is necessary to carry out plating at a higher plating rate by applying a higher current, i.e., at a higher current density. However, if plating is carried out under high-current density conditions using a conventional common plating apparatus and its operating method, the in-plane uniformity of a thickness of a plated film tends to become worse. The in-plane uniformity of the thickness of the plated film is required to be at a higher level than ever. Therefore, shortening the distance between a regulation plate and a plating object, as described in the patent document 2, is therefore important in carrying out plating under high-current density plating conditions.
It has been found by the present inventors that, when plating is carried out under high-current density conditions using a conventional common plating apparatus and its operating method, bumps formed by the plating tend to have a convex top, not a flat top. The formation of such convex-top bumps would cause the following problems: In WL-CSP (wafer level-chip size package) currently under development, after forming bumps by plating, the bumps are coated with a resin. If bumps have a convex top shape, an excess amount of resin must be applied to cover the entire bumps, leading to an increased cost. After the application of a resin, a surface of the resin is usually leveled with a spatula, called squeegee. A tall bump with a convex top can fall down upon leveling of the resin surface with a spatula (squeeze). After coating bumps with a resin, it is also a common practice to polish the resin and the bumps to a predetermined thickness by mechanical polishing. If the bumps have a convex top and therefore the resin has been applied in an excess amount, then the excess resin must be polished away, resulting in increased cost.
A plating apparatus and method has been proposed in which plating of a printed circuit board having through-holes is carried out while driving a pair of stirring bars, one at 5 cm/sec to 20 cm/sec and the other at 25 cm/sec to 70 cm/sec, in a plating solution (see, e.g., Japanese Patent Laid-open Publication No. 2006-41172, patent document 3). However, bumps having a flat top will not be formed if plating is carried out while moving a pair of stirring bars respectively at such a velocity.